The present invention relates generally to manufacture and repair of machine parts, and, more specifically, to surface finishing of such parts.
Machines are assemblies of various parts which are individually manufactured and assembled. Machines typically include metal parts, although synthetic and composite parts may also be used. And, each part requires specialized manufacturing.
For example, metal parts may be fabricated from metal stock in the form of sheets, plates, bars, and rods. Metal parts may also be formed by casting or forging. Such parts may be machined to shape in various manners.
Machining requires the selective removal of material to configure the part to its final shape and size within suitable manufacturing tolerances, typically expressed in mils, and with a suitable surface finish which is typically smooth or polished without blemish.
Each step in the manufacturing process of machine parts adds time and expense which should be minimized for producing a competitively priced product. It is desirable for each subsequent step in the manufacturing process to avoid damaging previously finished portions of the part which would then require additional corrective finishing steps.
Gas turbine engines are an example of a complex machine having many parts requiring precise manufacturing tolerances and fine surface finishes. A typical engine includes a multistage compressor for pressurizing air which is mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through one or more turbine stages that extract energy therefrom. A high pressure turbine powers the compressor, and a low pressure turbine provides output power, such as powering a fan disposed upstream from the compressor in an aircraft engine application.
The engine thusly includes various stationary components, and various rotating components which are typically formed of high strength, state of the art metal and composite materials. The various parts undergo several steps in their manufacturing and are relatively expensive to produce.
Of particular interest in manufacturing compressor and turbine rotor disks is maintaining smooth surface finish thereof and large radii along edges therein for minimizing stress during operation. Rotor disks support corresponding rotor blades around the perimeters thereof, and are subject to substantial centrifugal force during operation. The centrifugal force generates stress in the rotor disk which can be concentrated at sharp edges or small comers in the disk, which must therefore be suitably eliminated.
In one type of rotor disk, axial dovetail slots are formed through the perimeter of the disk for retaining rotor blades having corresponding axial dovetails. The dovetails include one or more pairs of dovetail tangs, in the exemplary form of a fir tree, which mate in complementary dovetail slots formed between corresponding disk posts.
The dovetail slots are typically manufactured by broaching wherein successively larger cutting tools cut the perimeter of the rotor disk to form the desired dovetail slots in a sequential operation. Each dovetail slot is broached in turn until the full complement of slots is formed around the perimeter of the disk.
The disk prior to the broaching operation has already undergone several steps in the manufacturing process including precision machining of most of its external surface. Broaching of the dovetail slots in the perimeter of the disk typically results in sharp corners or edges on the entrance side of the slot, and burrs on the exit side of the slot. The sharp entrance edges and burred exit edges require further processing to form suitably large radii which correspondingly reduce stress concentrations during operation of the rotor disk.
Deburring and radiusing of the rotor disk typically requires several additional processes in view of the complexity of the rotor disk and the complexity of the dovetail slots therein. For example, the individual rotor disk after broaching may be turned inside a bed of abrasive particles, such as the Sutton Blend (trademark) process, used to initially deburr the slots and form suitable corner radii therealong. However, the Sutton Blend process is directional and is effective for radiusing only some of the edges of the serpentine dovetail slots.
Accordingly, the disk undergoes additional processing for benching or further abrading slot edges, typically near their bases, by hand or robotically. One form of benching is conventionally known as Harperizing which is a trademark process using cloth wheels having abrasive therein.
This process is then followed by a conventional abrasive flow for blending the benched regions as required for achieving suitable radii.
These various steps require corresponding processing time, and are correspondingly expensive. And, hand benching always includes the risk of inadvertent damage to the rotor disk rendering it defective, and requiring scrapping thereof at considerable expense.
Furthermore, the rotor disk includes other machined features which may have sharp edges and burrs thereon which also require processing. For example, an annular row of axial holes extend through the web of the disk below the dovetail slots which receive retaining bolts during assembly. These bolt holes are suitably drilled, and like broaching, have sharp entrances and sharp exits with burrs thereon. These edges are also suitably radiused using the processes described above, which adds to the time and expense for disk manufacture.
The deburring and radiusing processes described above are used selectively for the edges being treated to avoid or minimize any changes to the remaining surface of the rotor disk which is typically smooth with a fine surface finish. Any damage to that finish requires additional processing and corresponding time and expense.
Accordingly, it is desired to provide an improved process for selective surface treating a workpiece, having little or no adverse effect on adjoining surface finish thereof.
A workpiece includes an aperture bordering a surface at a shifting edge. A stream of pliant shot is discharged in a carrier fluid at the edge with a shallow angle of incidence. And, the workpiece is rotated to sweep the shot stream along the shifting edge for abrasion thereof.